For several reasons it is desirable to use daylight as much as possible for providing lighting inside buildings. One reason is to save electrical energy and thereby save the environment from carbon dioxide, since a lot of, or at the present date, most, of the electrical energy produced in the world comes from some kind of combustion.
Another reason is that daylight is desirable before electrical lighting by humans in most situations. This has been shown in research as well as in observations of human behaviour and the arrangement of office space. Windows that admit daylight in buildings are important for the view and connection they provide with the outdoors. Daylight is also important for its quality, spectral composition, and variability. Indoor daylight is desired to be able to see well, and to experience environmental stimulation. Working long-term in electric lighting may affect health while working by daylight is believed to result in less stress and discomfort. Daylight provides high illuminance and permits excellent colour discrimination and colour rendering. Daylight may, however, also lead to uncomfort due to glare.
It is thus desirable to distribute daylight into buildings as long as glare is avoided. The traditional way to distribute daylight is to have windows of glass, allowing visible daylight to pass through the window and illuminate the room inside the window. If glare is produced by too direct or bright sunlight the traditional way has been to use shades to absorb or reflect some of the incident daylight and thereby reduce the light inside the room. A problem is then that the amount of light that is transmitted into the room often is too small to be sufficient and electrical lighting has to be used as compensation. Another way to reduce glare is to use frosted or structured glass in the window to scatter the incident light in all directions. In strong sunlight this solution, however, still produces too much glare. A scattering surface also reduces the transparency of the window so that it is useless for viewing through.
Another solution to reduce glare has been to have redirecting surfaces in all or part of the window, redirecting some of the incident light towards the ceiling of the room, e.g. by arranging prisms inside the window. Prism structures for light redirection are well known as well as a double rotating prism structure for optimal light redirection of (time varying) direct sunlight, see e.g. U.S. Pat. No. 5,729,387. Using prisms also reduce or destroy the ability to see through the window, affecting the function of the window to view through. Another problem using prisms inside the window is that the illumination of the ceiling will change as the sun angle shifts during the day if not active movement of the prisms is achieved to follow the sun movements.
Some rooms in a building are so large that light from windows do not reach all parts, and some rooms deep inside buildings does not have any walls to the outside to let daylight in through. In these cases systems have been developed to use optical fibres or light ducts to transport light deep into the building. A light duct is a pipe having strongly reflective inner walls. A system of light ducts is for example shown in the source book “Daylight In Buildings” published by the International Energy Agency (IEA). In these systems the collecting side of the duct is often equipped with collecting optics, often placed in a favourable angle to the sun. The collection systems are, however, bulky and not favourable from an esthetical point of view, leading to a resistance to them from architects. The bulky construction may also be in the way of other equipment wanted on the building.
There is thus a need to have a more efficient way of directing daylight into buildings through windows and light ducts in an efficient way while keeping cost down and producing a solution that is attractive or discrete enough so that people, and especially architects, want to use them.